1. Field of Invention
This invention pertains to telecommunications, and particularly to the handling of cells in a switching node of a telecommunications network operating in the asynchronous transfer mode.
2. Related Art and Other Considerations
The increasing interest for high band services such as multimedia applications, video on demand, video telephone, and teleconferencing has motivated development of the Broadband Integrated Service Digital Network (B-ISDN). B-ISDN is based on a technology know as Asynchronous Transfer Mode (ATM), and offers considerable extension of telecommunications capabilities.
ATM is a packet-oriented transfer mode which uses asynchronous time division multiplexing techniques. Packets are called cells and have a fixed size. An ATM cell consists of 53 octets, five of which form a header and forty eight of which constitute a "payload" or information portion of the cell. The header of the ATM cell includes two quantities which are used to identify a connection in an ATM network over which the cell is to travel, particularly the VPI (Virtual Path Identifier) and VCI (Virtual Channel Identifier). In general, the virtual path is a principal path defined between two switching nodes of the network; the virtual channel is one specific connection on the respective principal path.
At its termination points, an ATM network is connected to terminal equipment, e.g., ATM network users. Between ATM network termination points are a plurality of switching nodes having ports which are connected together by physical transmission paths or links. In traveling from an origin terminal equipment to a destination terminal equipment, ATM cells forming a message may travel through several switching nodes.
A switching node has a plurality of ports, each of which is connected by a link circuit and a link to another node. The link circuit performs packaging of the cells according to the particular protocol in use on the link. A cell incoming to a switching node may enter the switching node at a first port and exit from a second port via a link circuit onto a link connected to another node. Each link can carry cells for a plurality of connections, a connection being a transmission between a calling subscriber or party and a called subscriber or party.
Many cells which are transmitted through an ATM network travel from an origination node to a single destination or target node, and accordingly are known as point-to-point cells. Some switching nodes are capable of handling cells, known as point-to-multipoint cells or multicast cells, which travel from an origination node to a plurality of destination nodes. Some of the point-to-multipoint cells, although being for differing connections, may travel on the same link.
The switching nodes each typically have several functional parts, a primary of which is a switch core. The switch core essentially functions like a cross-connect between ports of the switch. Paths internal to the switch core are selectively controlled so that particular ports of the switch are connected together to allow a message ultimately to travel from an ingress side of the switch to an egress side of the switch, and ultimately from the originating terminal equipment to the destination terminal equipment.
In a conventional switching technique, queues or buffers are provided for each port for e.g., storing cells prior to readout. In situations in which cells may have one of a plurality of priority classes, each port may have a number of queues or buffers corresponding to the number of priority classes. Cells are fed into an appropriate buffer by an input queue selector, and are readout of the buffer at an appropriate time by an output queue selector.
The handling of point-to-multipoint cells, e.g., multicast cells, on an egress side of the switch, has received attention. See, for example, U.S. patent application Ser. No. 08/893,677 filed Jul. 11, 1997, entitled "BUFFERING OF POINT-TO-POINT AND/OR POINT-TO-MULTIPOINT ATM CELLS", which is incorporated herein by reference. Multicast encompasses the notions of both physical multicast and logical multicast. By "physical multicast" is meant that a cell is copied to different physical outputs. By "logical multicast" is meant that a cell is copied one or more times to the same physical output, each copy having its own VPI/VCI value. Physical and logical multicast may be combined in the same port.
In multicast terminology, a "leaf" is a connection to which a cell should be copied, and a "logical leaves" are connections in the same link (e.g., physical output) to which the cell should be copied. Concerning the egress side of a switch, if one leaf has read all the cells currently residing in an egress-side cell buffer that the leaf is supposed to read, that leaf is said to be "passive". On the other hand, when the cell buffer currently contains more cells that the leaf is supposed to read, the leaf is said to be "active".
Various multicast techniques are described in U.S. Pat. No. 5,572,522 to Calamvokis et al., entitled "ASYNCHRONOUS TRANSFER MODE SWITCH WITH MULTICASTING ABILITY", also incorporated herein by reference. In one of those techniques, each arrival of a new cell from the switch core causes a multicast expander to queue associated output VCNs that are not already in a scheduling loop. However, neither addition nor removal of output VCNs are contemplated, as it is assumed that all output VCNs are set up at the same time and remain for the entire duration of a connection. Another multicast technique is described in U.S. Pat. No. 5,528,588 to Bennett et al.
What is needed, therefore, and an object of the present invention, is method and apparatus for efficiently handling active and passive leaves and connections, and for adding and removing leaves during an existing connection.